1. Field of the Invention
This invention relates to a bias voltage supply circuit, and more particularly, to a semiconductor bias voltage supply circuit for providing an electrical bias with a constant output voltage characteristic little affected by output current.
2. Description of the Prior Art
Constant bias voltage supply circuits are very useful in integrated circuit (IC) design. Many forms of constant bias voltage supply circuits have been developed. In constant bias voltage supply circuits, it is required that the operating bias voltage is not changed by a variation in the output current.
As is well known, a principal example of a bias voltage supply circuit in common use in the transistor circuit is shown in FIG. 1. In FIG. 1, DC power supply terminal 10 is connected to the positive terminal of battery 12, with its negative terminal connected to reference potential terminal 14. DC power supply terminal 10 is connected via DC current source 16, to diode 18 and resistance 20. Output terminal 22 is connected to a circuit connection between DC current source 16 and diode 18. Load 24 is connected between output terminal 22 and reference potential terminal 14. In this case, diode 18 can be a resistor or a constant-current circuit including a transistor and resistors.
If the output current of DC current source 16 is taken as I.sub.1, the load current flowing to load 24 as I.sub.out, and the forward voltage of diode 18 as V.sub.f, output voltage V.sub.out is as follows. EQU V.sub.out =V.sub.f +(I.sub.1 +I.sub.out)R.sub.1 ( 1)
where, R.sub.1 is the resistance of resistor 20.
If, in this formula, the forward current of diode 18 is taken as I.sub.d1, and its reverse saturation current as I.sub.sd1, the following is obtained. ##EQU1## where, V.sub.t : thermal voltage=kT/q,
k: Boltzmann's constant, PA1 T: absolute temperature, PA1 q: electron charge.
Then, equation (1) is transformed as follows. ##EQU2##
As an example, assume that when the ambient temperature is 27.degree. C., V.sub.t =26 mV. The output voltage V.sub.out(0) when there is no load (i.e., when I.sub.out =zero amperes), is then calculated as follows from equation (2) above, microamps, assuming that I.sub.1 =100, R.sub.1 =3000 ohms and I.sub.sd1 =2.3.times.10 microamps: ##EQU3##
Next, if output voltage V.sub.out(50) when there is a load current of 50 .mu.A is calculated from formula (1), we have the following. ##EQU4##
In other words, whereas output voltage V.sub.out(0) when there is no load is 1.000 V, output voltage V.sub.out(50) when there is a load current of 50 .mu.A, which is half of current I.sub.1 of DC current source 14, is 0.832 V, representing a fall of 0.178 V. Thus the bias voltage supply circuit of FIG. 1 causes fluctuations in the load current which give rise to large fluctuations in the output voltage V.sub.out.
Another example of a prior art bias voltage supply circuit is shown in FIG. 2, in which transistors 26 and 28 are used. In this case, considering that the relationship of base current I.sub.b1 of transistor 26 and load current I.sub.out is I.sub.b1 I.sub.out, and taking the base-emitter voltage, emitter current, and reverse saturation current of transistor 26 as V.sub.be1, I.sub.e1, and I.sub.s1 respectively, output voltage V.sub.out is as follows. ##EQU5##
If the grounded emitter circuit current amplification factors of transistors 26 and 28 are taken as .beta..sub.1 and .beta..sub.2 respectively, we have the following. EQU I.sub.e1 =I.sub.c1 +I.sub.b1 =I.sub.c1 .multidot.(1+1/.beta..sub.1)
wherein I.sub.c1 =I.sub.1 -I.sub.b2, then EQU I.sub.e1 =(I.sub.1 -I.sub.b2).multidot.(1+1/.beta..sub.1)=I.sub.1 .multidot.(1+1/.beta..sub.1)-I.sub.b2 .multidot.(1+1/.beta..sub.1)
therefore, ##EQU6##
If .beta..sub.2 is assumed to be very large, then we have EQU I.sub.e1 =I.sub.1 .multidot.(1+1/.beta..sub.1) (4)
with emitter current I.sub.e1 ceasing to be influenced by load current I.sub.out. This being so, the output voltage V.sub.out expressed by formula (4) given above also ceases to be affected by load current I.sub.out.
In the bias voltage supply circuit of FIG. 2, for the formula (4) to be applicable, grounded emitter circuit current amplification factor .beta..sub.2 of transistor 28 must be very large. In other words, the magnitude of the changes in output voltage V.sub.out of the bias voltage supply circuit of FIG. 2 in response to fluctuations in load current I.sub.out depends to a large extent on the current-amplification action of transistor 28. However, for transistor 28 to provide this current-amplification action, the base-emitter junction of transistor 28 must be biased in the forward direction. This means that voltage V.sub.cc at DC power supply terminal 10 must be EQU V.sub.cc =V.sub.out +V.sub.be2
(where V.sub.be2 is the base-emitter voltage of transistor 28). This requires a high consumption of electric power. It may be noted here that voltage V.sub.cc of DC power supply terminal 10 in the bias voltage supply circuit of FIG. 1 need by only V.sub.cc =V.sub.out.